Trout feed when there is food to be eaten. The amount they eat depends, of course, on their size, availability of food, and on water temperature. Water temperature affects their metabolic rate and desire or need for food. And presumably their eating habits depend on things we don't understand. Temperature is certainly something that we should not get too anal about ... but still it's interesting.

According to many sources, each species of trout has an ideal temperature range in which it feeds the heaviest. But this is likely of more academic interest than practical use to most of us out for the day or away on vacation. If you've just driven across two provinces (or three states) to fish in a famous river and the water temperature is outside of the reported ideal range what are you going to do? Drive home? No. You are going to fish and make the best of it. Perhaps the temperature may explain why we are not hammering them, but what's the sense in moaning about it - eh?

A few years ago I met a man from Rhode Island sitting on a bridge crossing the North Raven River in central Alberta. He was eating a cigar (yes, eating the cigar) and generally taking it easy while waiting for his partner to come off the stream. I had just stopped by the stream to kill an hour or two in the afternoon while waiting for the main event later that evening on another nearby river. (This was the afternoon matinee. Tonight was the full-length feature.)

Neither of us had caught fish in the couple of hours we were both on the water. It was a nice day. He was heading elsewhere the next day and I was anticipating the night shift. We were happy as clams.

Out of the willow-tangled meadows emerged a local angler all bedecked in the latest gear - right out of the catalogues. He inquired how we had faired and then told us he had caught only seven browns and the fish were decidedly off compared to a few days before when he had caught many more. He told us the water temperature was way down and well below the ideal brown trout range and he might as well go home. Clearly not an option for the cigar eater or me.

The survival range for trout is published at 35 to 75 °F (2 to 23 °C). And the optimum feeding range for most trout species is between about 50 and 68 °F (10 to 20 °C). Cutthroats and brook trout feed optimally at slightly cooler temperatures.

I am doubtful about the reported optimum feeding temperature ranges for trout. On-stream experience does not support the 'facts.' My brother, Gary, has a theory about feeding and temperature: the trend in temperature direction is more important than the actual temperature. At the lower range, he thinks that the exact water temperature is not as important as whether it's getting warmer or colder. If the stream temperature rises from 45 to 50 °F (7 to 10 °C), the trout will get jazzed and increase feeding - perhaps not as aggressively as they might at, say, 60 °F. But if the temperature drops from 55 down to 50 °F (13 to 10 °C) they will go off their feed. So you could have two identical water temperatures in the same creek, perhaps two or three days apart, and the feeding habits could be totally different. It depends on whether the water is getting warmer or colder.

Of course this is complicated by the relative insect activity as well. The bugs may also be turned off by falling water temperatures and therefore will not be as active. So, does trout feeding increase (as the temperature goes up) because they get hungry? Or because there is simply more bug activity?

Gary's theory may or may not be so, but on-stream experience seems to bear this out - sometimes. Trout feed better when the temperature is on the upswing (at the lower range) than they do when the water temperature is falling.

Higher temperatures definitely cause feeding activity to drop off. Trout fishing at lower elevations (and in southern latitudes) can be quite poor when there is a long hot spell. If the water temperature gets too high, the dissolved oxygen content can fall to fatal levels.

There's a final issue about the published temperature and activity ranges for trout. They just don't hold true at the lower end. They imply that trout simply will not eat when the water temperature hovers just above freezing. So why then are we able to catch trout in western streams from November through March when the water temperature is a degree or so above freezing? Sure the fish are sluggish, but they do eat our flies, and I have seen rainbows actually chase nymphs in ice-cold water. (And of course, the reference I found - stating the minimum temperature for survival is 35 °F - is simply wrong. River water can be supercooled to a half degree or so below freezing. The fish are sluggish, but they don't die.)

It gets more interesting yet. Hourly catch rates in winter when the water temperature is barely above freezing are often higher than in spring and summer. Go figure.

Clive Schaupmeyer is an outdoor writer and photographer. He is the author of
The Essential Guide to Fly-Fishing,
a 288-page book for novice and intermediate fly anglers. His photo of a boy
fishing was judged the best outdoor picture of 1996 published by a member of
the Outdoor Writers of Canada. He fly-fishes for trout in Alberta's foothill
and mountain streams and for pike near his home in Brooks, Alberta.